Servicing of landing gear shock absorbers

ABSTRACT

A method and portable apparatus for servicing a shock absorber on a landing gear assembly of an aircraft in a weight-on-wheels state is disclosed. The shock absorber includes at least one chamber containing both hydraulic fluid and a gas in fluid communication with each other. The apparatus includes a source of gas and a source of hydraulic fluid. The amount of hydraulic fluid in the chamber is corrected, preferably such that the chamber is then filled with a known amount of degassed hydraulic fluid. A pre-set mass of gas is then delivered into the chamber under the control of a gas delivery system of the portable apparatus. More accurate servicing of a shock absorber may thus be provided since account is additionally taken of gas dissolved in hydraulic fluid. By delivering a pre-set mass of gas into the chamber, there is no need to rely on a measure of gas pressure or H-dimension (h) when servicing the shock absorber.

RELATED APPLICATIONS

The present application claims priority from Great Britain ApplicationNumber 1512144.5, filed Jul. 10, 2015, the disclosure of which is herebyincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention concerns the servicing of aircraft landing gearshock absorbers. More particularly, but not exclusively, this inventionconcerns a method of servicing a shock absorber on a landing gearassembly of an aircraft, and an apparatus for servicing such a shockabsorber. The invention also concerns an associated control unit for usein such a method or apparatus and an associated computer programproduct.

Aircraft landing gear assemblies typically include a shock absorberassembly (for example of the type referred to as an oleo strut) whichprovides suspension and shock absorbing functions for the aircraftduring landing and take-off. An aircraft 101 including such shockabsorber assemblies 102 is shown in FIG. 1. A single such shock absorberassembly 102 is shown in greater detail in FIG. 2 together with aschematic illustration of the wheels 109. (Parts of the landing gearincluding, for example, the axles for mounting the wheels and the upperpart of the landing gear for receiving the oleo strut, have been omittedfrom FIG. 2 for the sake of clarity.) The shock absorber 102 comprises apiston 104 received within a cylinder 103, as is conventional with suchshock absorbers. The cylinder 103 and piston 104 are connected via atorque link 107 for inhibiting rotation of the piston 104 relative tothe cylinder 103, as is well known to those skilled in the art. Theshock absorber 102 typically includes one or more gas springs (not shownseparately in FIG. 2) with damping provided by hydraulic fluid (also notshown separately in FIG. 2). Over time, as the shock absorber 102 isused on successive landings and take-offs, a proportion of the gasand/or the hydraulic fluid escapes. It is important to ensure that thelevels of both the gas and the hydraulic fluid are correct, as otherwiseperformance of the shock absorber may be adversely affected. Regularservicing of the shock absorber is thus required to ensure that the gasand fluid levels in the shock absorber are maintained at suitablelevels.

Accurate determination of the levels of the gas and the hydraulic fluidis not however a straightforward process. The amount of gas in the shockabsorber is typically measured by means of measuring the extension ofthe shock absorber relative to a reference point (often referred to as‘H-dimension’) with the weight of the aircraft being supported by thelanding gear assemblies of the aircraft. FIG. 3 shows an H-dimension(labelled as “h”) as being measured between the bottom of the slidingcylinder 103 and the top of the attachment lug of the lower torque link107 of the shock absorber 102. The temperature and pressure of the gasare then used to assess, using a look-up chart, whether the“H-dimension” indicates that there is an appropriate amount of gas inthe shock absorber. The gas temperature thus needs to be measured. Oftena sensor measures the gas pressure, which is of course proportional tothe load supported by the shock absorber. Thus, according to a typicalservicing procedure, the pressure and temperature are ascertained andthe H-dimension of the shock absorber is measured; if, for that givenpressure and temperature, the H-dimension is below a defined limit, thenone of the following maintenance actions takes place:

-   -   1. Weight on wheels service: The shock absorber is inflated with        gas (typically Nitrogen, N₂) to raise the H-dimension of the        gear to above the limiting threshold.    -   2. Weight off wheels service: The aircraft is jacked and the        shock absorber is depressurised. The shock absorber is redressed        with hydraulic fluid and subsequently charged with N₂ to a        specified pressure.

The use of the H-dimension to assess incorrect levels of gas andhydraulic can suffer from inaccuracies resulting from the frictionbetween the moving parts of the shock absorber.

There are also disadvantages with both of the abovementioned proceduresfor servicing of landing gear shock absorbers. With a weight on wheelsservice, it is often assumed that the reduction in the H-dimension isthe result of leakage of N₂ alone from the shock absorber. There areother reasons why the H-dimension might be lower than desired, includingfor example the loss of hydraulic fluid. Whilst some shock absorberdesigns permit weight on wheels servicing of both N₂ and hydraulicfluid, such a servicing process is in practice a complicated anddemanding process for a maintenance engineer, and one which can increasethe risk of the shock absorber being serviced with relatively lowaccuracy. The process may take one or two hours per shock absorber. Aweight off wheels service on the other hand is more likely to result inan accurate and correct refilling of the shock absorber with thehydraulic fluid and gas. However, a weight off wheels service involvesjacking-up of the aircraft and can take one or two days to complete.This is costly to the operator as the aircraft will be taken out ofrevenue service whilst the maintenance action is carried out. Thus,whilst a weight-on-wheels service is fast, the confidence ofrefilling/topping up the levels of hydraulic fluid and gas accurately tothe correct level is not as great as the much slower and more costlyweight-off wheels service. In both cases, the maintenance engineer mayhave to follow a large number of manual actions to complete the service.There is therefore an associated risk of human error resulting in anincorrectly serviced gear.

GB2514336 describes a method for carrying out a weight-on-wheels serviceof a landing gear shock absorber, in which the levels of damping fluidand gas are ascertained and then adjusted as required using varioussensors and calculations. It is believed that further improvement ishowever possible.

SUMMARY OF THE INVENTION

The present invention provides, according to a first aspect, a method ofservicing a shock absorber on a landing gear assembly of an aircraft,whilst the weight of the aircraft is at least partially supported by thelanding gear assembly (i.e. whilst the aircraft is in a weight-on-wheelsstate). The method has application in relation to a shock absorbercomprising at least one chamber containing both hydraulic fluid and agas in fluid communication with each other. The method comprises a stepof exhausting all gas from the chamber. The method comprises a step ofdegassing at least some dissolved gas from the hydraulic fluid. Themethod comprises a step of ensuring that one or more set criteria, whichdepend on the amount of hydraulic fluid in the chamber, are met. Themethod comprises a step of adjusting the amount of hydraulic fluid inthe chamber, so that such criteria are met (for example topping up thehydraulic fluid if so required). The method comprises a step ofdelivering a pre-set mass of gas into the chamber of the shock absorber.

It will be understood that the hydraulic fluid is a liquid. Gas in theshock absorber may dissolve in the hydraulic fluid and/or be releasedfrom it at a rate dependent on various conditions, such as for examplethe temperature and pressure of the gas, the temperature of the liquid,the concentration of gas already in solution, the amount by which thehydraulic fluid has been agitated and the type of shockabsorbing/suspension function the shock absorber has recently performed.For example, when an aircraft lands the pressure in the shock absorberswill increase rapidly to support the weight of the aircraft; in chamberswhere gas and hydraulic fluid are unseparated, gas will be absorbed intothe hydraulic fluid. This process can be accelerated by vigorous mixingof the fluid during the landing event and subsequent taxiing period.Nitrogen absorbed in hydraulic fluid when the gas in the shock absorberis under pressure (for example as is typically the case in a weight onwheels state) may be released when the weight is off the wheels, forexample after take-off Thus during flight the mass of gas (the gas notcontained within the hydraulic fluid) in the shock absorber may increaseas dissolved gas degasses from the hydraulic fluid. Taking into accountthe pressure, temperature, and volume of the gas (not contained withinthe hydraulic fluid), in the weight on wheels state, without consideringthe amount of gas contained within the hydraulic fluid (whetherdissolved, suspended as bubbles of gas in the liquid, or otherwisecontained) will not provide a very accurate determination of the sumtotal of mass of gas within the shock absorber and could provideunderestimates of the amount of gas that is contained in the shockabsorber and therefore underestimates of the pressure of the gas thatwill be in the gaseous state for a given temperature and volume in theshock absorber immediately before landing. It will be appreciated thatthe degassing of at least some dissolved gas from the hydraulic fluidmay include releasing gas by means of desorption so that some gas comesout of solution and/or may include releasing gas bubbles from within thehydraulic fluid by means of such bubbles releasing their gas from withinthe body of liquid to outside the body of liquid.

In an embodiment of the invention, the level of hydraulic fluid in theshock absorber is measured only once gas held in solution in thehydraulic fluid is released so that the volume of fluid as measured iscloser to a true representation of the amount of hydraulic fluid in theshock absorber and is not artificially higher as a result of dissolvedgas or gas bubbles in the fluid. Also, if the amount of gas held in thefluid is lower than it would otherwise be, it is safer to assume that nosignificant mass of gas will degas when the shock absorber extends oncethe weight is off the wheels (for example, when in flight). It cantherefore be more justifiably assumed that the amount of gas introducedinto the shock absorber will represent the amount of gas that is in theshock absorber in the gaseous state immediately before landing. Incertain methods of the prior art, a less than expected H-dimension wouldbe interpreted only as a lack of gas, leading to the replacement of losthydraulic fluid and/or absorbed gas with additional gas, most probablyleading to inaccurate servicing of the shock absorber. Also, in suchprior art methods, if adding more gas whilst the shock absorber is underpressure (in a weight on wheels configuration for example) no accountwould be taken of the absorption of gas during the pressurisationprocess, also leading to greater potential inaccuracies. In certainembodiments of the invention, it is no longer necessary to measure theH-dimension when maintaining/servicing a shock absorber.

The shock absorber may be the principal shock absorber of the landinggear assembly. For example, the landing gear assembly may comprise astrut member for carrying the majority of the load to be carried by thelanding gear assembly when the aircraft is in the weight-on-wheelsstate. The shock absorber may be in the form of an oleo strut. The shockabsorber may be in the form of a single-stage shock absorber. Such asingle-stage shock absorber may comprise a single chamber within whichboth the gas and hydraulic fluid are accommodated. The shock absorbermay be in the form of a multiple-stage shock absorber. Such amultiple-stage shock absorber may comprise multiple chambers withinwhich gas and/or hydraulic fluid are accommodated. For example there maybe a first chamber containing gas and hydraulic fluid with the gas beingat a lower pressure when the aircraft is in the weight-off-wheels stateand a second chamber containing gas (but possibly not hydraulic fluid)with the gas being at a higher pressure when the aircraft is in theweight-off-wheels state. Such multiple chambers are typically providedin series so that the higher pressure stage contributes most to thesuspension of the aircraft under high loading. It may be that said atleast one chamber comprises two or more separately discerniblecompartments which are in fluid communication with each other.

The shock absorber may be one of multiple shock absorbers associatedwith the same single landing gear assembly. For example, the landinggear assembly may comprise twin struts, arranged in a parallelconfiguration, each strut comprising a separate shock absorber accordingto the present invention.

The shock absorber may comprise multiple chambers. The chambers may beseparate from each other; that is, not in fluid communication with eachother. The shock absorber may be part of a landing gear strut. The shockabsorber may be in the form of a multiple-stage, for example atwo-stage, shock absorber.

It will be understood that the step of exhausting all gas from thechamber means exhausting substantially all of the gas which is not atthat time dissolved or otherwise contained within the hydraulic fluid.It will also be understood that, in certain embodiments, there mayremain one or more empty spaces in the chamber not occupied by thefluid. If one or more such spaces exist, there will inevitably be somegas contained in the space, because a perfect vacuum will not be formed.It will however be the case that any gas remaining outside of thehydraulic fluid but within the space, after the step of exhausting allgas from the chamber has been completed, will as a practical matter benegligible. The space left will be at a pressure significantly lowerthan atmospheric pressure.

The shock absorber will typically (i.e. but not necessarily) be arrangedsuch that it is a generally elongate structure, typically aligned withthe direction of loading. The shock absorber will typically be arrangedsuch that its length changes between an extended (e.g.weight-off-wheels) configuration and a contracted (e.g. typicallyweight-on wheels) configuration.

The step of exhausting all gas from the chamber might, depending on thedesign of the landing gear assembly and unless other steps are taken,mean that the shock absorber closes such that it moves in a directionfrom an extended configuration to a contracted configuration. Thechamber may reduce in volume as the shock absorber closes such that itmoves in a direction from an extended configuration to a contractedconfiguration.

It will typically be the case that during the step of exhausting all gasfrom the chamber, the volume of the chamber reduces so that the shockabsorber contracts. The method may further include a step of using astop (for example in the form of a collar) which is arranged to resistfurther contraction of the shock absorber. The stop may in use bear atleast some of the load that would otherwise be sustained by the shockabsorber.

It may be that a collar is arranged around the landing gear assembly,the collar bearing at least some of the load that would otherwise besustained by the shock absorber. The collar may thus perform thefunction of a stop. The collar may assist in the supporting, at least inpart, of the weight of the aircraft. The step of arranging such a stop,or collar, may be conducted before the step of exhausting all gas fromthe chamber is completed. The step of arranging such a stop, or collar,may be conducted before the step of exhausting the gas from the chamberis started. The collar may extend more than 180 degrees around the shockabsorber.

There may be one or more such stops. It may be that substantially all ofthe vertical load that is sustained by the landing gear assembly istransmitted via said one or more such stops. The one or more such stopsmay be removable parts. The servicing method may therefore include astep of removing the stop from the aircraft (i.e. the stop is removedbefore the aircraft commences either taxiing manoeuvres or take-off). Asmentioned above, the stop may be in the form of a collar.

Using a stop, such as a collar for example, which allows the aircraft tobe maintained in a weight-on-wheels state allows emptying of the shockabsorber under lower pressures. Existing aircraft have valves whichallow gas to be expelled with weight on wheels but do not permit removalof hydraulic fluid, except under pressure and in the context of addingnew fluid whilst existing fluid is removed (i.e. flushing through).Flushing-through hydraulic fluid is an inefficient process.

The step of degassing at least some dissolved gas from the hydraulicfluid may include a step of actively lowering the pressure within thechamber, for example by applying a partial vacuum. A vacuum pump may beused. The step of exhausting all of the gas may include a step ofactively lowering the pressure within the chamber. A vacuum pump may beused both to exhaust gas from the chamber and to degas dissolved gasfrom the hydraulic fluid at the same time. It will be appreciated thatin some embodiments, the steps of degassing dissolved gas and ofexhausting gas from the chamber may be performed simultaneously, atleast partially (one step beginning before the other finishes).

It may be that the step of exhausting all of the gas includes releasingat least some, and possibly all of, the exhausted gas to atmosphere. Itmay be that the step of exhausting all of the gas includes capturing atleast some of gas so exhausted, for example in a tank.

The step of exhausting all of the gas from the chamber may include usinga fluid line along which gas flows in one direction. The step ofdelivering the pre-set mass of gas into the chamber may include usingthe same fluid line but with the gas flowing in the opposite direction.It may be that the fluid line remains connected (and is therefore notdisconnected) between the step of exhausting all of the gas from thechamber and the step of delivering the pre-set mass of gas into thechamber.

It may be that the step of degassing at least some dissolved gas fromthe hydraulic fluid includes a step of releasing and capturing at leastsome of the gas.

It may be that the step of delivering a pre-set mass of gas into thechamber includes delivering gas from a variable volume tank. It may bethat the volume, pressure and temperature of the gas inside thevariable-volume tank are each ascertained so that the mass of gas in thetank can be calculated. The method may include using measures of thevolume, pressure and temperature of the gas inside the tank at at leasttwo different times as the volume of the tank reduces in order toascertain the mass of gas delivered from the tank (e.g. in order tocorrectly deliver the correct mass of gas). It may for example be thecase that the mass of gas in the tank is ascertained, that the mass tobe delivered to the shock absorber is subtracted from the mass of gas inthe tank to provide a target value, and that the gas in the tank is thengradually (e.g. continuously and progressively) delivered to the shockabsorber until such time as the mass of gas left in the tank is equal tothe target value. The pressure, volume and temperature of the gas in thetank, as gas is being delivered to the chamber of the shock absorber, isascertained many times during the process such that the moment at whichthe target value is reached can be accurately ascertained. An actuatormay be controllable to adjust the volume of the tank so as to force gasout of the tank into the chamber. The actuator may be hydraulicallypowered. It may be that the gas in the tank is effectively injected intothe chamber by such an actuator. The volume of the variable volume tankmay reduce by more than half, and possibly more than 75%, whendelivering gas to the chamber. It may be that the volume of the variablevolume tank is reduced by more than 99%, so that the volume of the tankis close to zero when the chamber has been filled with the pre-set massof gas. In such a case, the tank may be filled (before the step ofdelivering the pre-set mass of gas into the chamber) from a gas sourceso that the amount of gas in the tank is equal to the pre-set mass.Thus, there may be a first measurement (at a first time) of the volume,pressure and temperature of the gas inside the tank which relates to amass that is equal to the pre-set mass, so that the second measure (atthe second time) is simply determining when the volume of the tank isreduced to zero. It may be that the step of delivering a pre-set mass ofgas into the chamber includes measuring out one or more known masses ofgas and delivering said one or more known masses of gas, successively,into the chamber. It may be that a single pre-set mass of gas iscreated, per separate chamber of the shock absorber, and that the singlepre-set mass is then injected into the respective shock absorberchamber.

The step of delivering a pre-set mass of gas into the chamber mayinclude delivering the pre-set mass via a regulator that is configuredto measure or regulate the rate of injection of mass of gas per unittime. There may be a control unit which monitors the sum total of massof gas so delivered over time and stops the delivery of gas once thepre-set mass of gas to be delivered is reached. If the rate of mass flowis precisely known then it may be possible to deliver the pre-set massof gas into the chamber by means of delivering gas at the known rate fora pre-set period of time.

It may be that the step of ensuring said one or more set criteriadepending on the amount of hydraulic fluid in the chamber are metcomprises testing whether the hydraulic fluid is at a given level (e.g.a given volume) and, if not, adjusting the level accordingly. In thecase where a collar is arranged around the landing gear assembly, thecollar may set a predetermined level of hydraulic fluid, as a result offixing a fixed extension of the length of the shock absorber. The levelof hydraulic fluid may be set by an outlet of the shock absorber fromwhich fluid may be expelled moving relative to the level of the fluid asthe length of the shock absorber contracts, the outlet thus beingpositioned in a known fixed position when the further contraction of thelength of the shock absorber is stopped by the collar.

It may be that the steps of the method are performed automatically. Themethod may for example be in the form of a semi-automated method ofservicing the shock absorber. There may for example be manual steps inaddition to automated steps. There may be a manual step of making aconnection, for example with a hose or the like, between a part ofautomated servicing equipment and a part of the shock absorber. Theremay be a manual step simply of providing, for example transporting tothe aircraft, apparatus suitable for performing the steps of the methodof the present invention. Two or more, preferably all, of the followingsteps are automated:

-   -   exhausting all gas from the chamber,    -   degassing at least some dissolved gas from the hydraulic fluid,    -   ensuring that one or more set criteria, which depend on the        amount of hydraulic fluid in the chamber are met, including if        so required adjusting the amount of hydraulic fluid in the        chamber    -   delivering a pre-set mass of gas into the chamber.

It may be that each such step is automated in the sense that a piece ofapparatus (preferably the same set of equipment) performs the step. Itmay be that the steps are automated in the sense that the steps areautomatically conducted one after the other.

The apparatus used in performing the method may comprise a source ofgas, a source of hydraulic fluid, and a gas delivery system. Theapparatus may be in the form of the apparatus according to any aspect ofthe present invention as claimed or described herein, including anyoptional features relating thereto.

The method may include the use of a control unit. The control unit maybe configured to control processes effected by the method including oneor more of the step of exhausting gas from the chamber, the step ofdegassing dissolved gas from the hydraulic fluid, the step of ensuringthat said set criteria are met, and the step of delivering a pre-setmass of gas into the chamber. It may be that all such steps aremonitored, effected or otherwise controlled by the control unit. It maybe that, during at least one of the steps of the method, for example thestep of delivering a pre-set mass of gas into the chamber, the controlunit monitors one or more inputs, for example including one or moreinputs concerning temperature and pressure measurements from appropriatesensors, for conditions suggestive of a fault. If such a fault issuspected by the control unit, an electronic fault-detected flag may beset by the control unit, or some other indication be provided, that canbe used to alert maintenance engineers of the likely fault. The controlunit on detecting a suspected fault may cease carrying out one or moresteps of the method and may, for example, stop delivering gas into thechamber. The conditions suggestive of a fault may for example be thatthe measured temperature and/or pressure is outside an expected range.

There may be a single control unit that performs all functions referredto above as being under the control of a control unit. Multiple controlunits, or computer processors, may be provided.

The present invention also provides an apparatus for performing a methodof servicing a shock absorber on a landing gear assembly of an aircraft,whilst the weight of the aircraft is at least partially supported by thelanding gear assembly. The apparatus may be configured for performing amethod according to any aspect of the present invention as claimed ordescribed herein, including any optional features relating thereto.

There may be provided a, preferably portable, apparatus for servicing ashock absorber on a landing gear assembly of an aircraft when theaircraft is in a weight on wheels configuration, the shock absorbercomprising at least one chamber containing both hydraulic fluid and agas in fluid communication with each other. Such an apparatus maycomprise a source of gas, a source of hydraulic fluid, and a gasdelivery system for delivering a pre-set mass of gas into the shockabsorber.

It will be understood that the pre-set mass of gas may be predeterminedto be all of the gas that is to be contained in said at least onechamber during operation of the aircraft. The pre-set mass of gas mayfor example be a fixed mass of gas that is representative of the sumamount of gas that should be contained in the said at least one chamberof the shock absorber after a correct service. It is the mass that ispre-determined, not the volume of gas supplied or the pressure to whichthe shock absorber is filled with gas.

It may be that the apparatus has two parts, namely a first part forexhausting gas from the chamber and a second part for delivering thepre-set mass of gas into the shock absorber. The first and second partsmay share components. It may be that the first part is substantiallyseparate from the second part.

The apparatus may further comprise a vacuum pump for exhausting gas fromsaid at least one chamber.

The apparatus may include a fluid trap for collecting hydraulic fluidthat may flow out of the chamber as the gas is exhausted.

The apparatus may further comprise a pump for delivering hydraulic fluidto the shock absorber from the reservoir.

The apparatus may be provided on a single servicing cart. Such aservicing cart may be arranged to transport all of the parts of theapparatus to and from an aircraft in a weight on wheels state.

The apparatus may include one or more collars, each collar beingarranged to stop contraction of a particular type of shock absorber andbeing able to sustain the compressive loads of the same magnitude asthose sustained by the shock absorber in the weight-on-wheels state.

It may be that the apparatus includes a gas port connector, for examplea quick-connector, for connecting to a gas port connector of the shockabsorber, via a hose for example. The apparatus may include a controlunit. The control unit may be configured to control delivery by the gasdelivery system of the pre-set mass of gas into the shock absorber. Thecontrol unit may be configured to control receiving of gas into thetank. The control unit may be configured to control exhausting of gasfrom the chamber. The control unit may be configured to controldegassing of dissolved gas from the hydraulic fluid in the chamber. Thecontrol unit may be configured to control deciding whether to adjust theamount of hydraulic fluid in the chamber. The control unit may beconfigured to control adjusting the amount of hydraulic fluid in thechamber if necessary. The control unit may be configured to controladding an amount of hydraulic fluid so that a target level of fluid inthe chamber is obtained.

The control unit may be configured to control all such things.

It may be that the gas delivery system for delivering a pre-set mass ofgas into the shock absorber comprises a regulator that is configured tomeasure or regulate the rate of delivery of mass of gas per unit time.

The gas delivery system may be controlled by a control unit thatmonitors the mass per unit time of gas flowing into the shock absorber,and from such information tracks the mass of gas delivered to thechamber. The control unit may determine when the sum mass of gasdelivered to the chamber has reached the target mass. The gas deliverysystem may measure the mass flow rate by using an orifice flow meter.Such an orifice flow meter may create a choked flow. The orifice flowmeter may utilise an orifice plate. The gas delivery system may measurethe mass flow rate by creating a choked flow, with means other than anorifice, for example by means of a restriction provided by a nozzle. Thecontrol unit may use at least three, and preferably all, of thefollowing four measurements in order to calculate the mass flow rate:temperature of gas upstream of choked flow region, pressure of gasupstream of choked flow region, temperature of gas downstream of chokedflow region, and pressure of gas downstream of choked flow region. Itmay be necessary to calibrate the set-up used to calculate the mass flowrate; or is may be that appropriate scaling factors can be calculated inadvance.

It will be understood that the pre-set mass of gas may be predeterminedto be all of the gas that is to be contained in said at least onechamber during operation of the aircraft.

It may be that the apparatus has two parts, namely a first part forexhausting gas from the chamber and a second part for delivering thepre-set mass of gas into the shock absorber. The first and second partsmay share components. It may be that the first part is substantiallyseparate from the second part.

The apparatus may include a tank which captures gas exhausted from thechamber. The apparatus may include a tank which delivers gas to thechamber. The same tank may be used to both capture gas exhausted fromthe chamber and deliver gas to the chamber. The tank may have a variablevolume. There may be an actuator arranged to effect a change in thatvolume. The operation of the actuator may be effected under control ofthe control unit. The control unit may be configured to control thereceiving of gas into the tank (including for example gas exhausted bythe shock absorber and/or the gas received from the source of gas). Thevolume of the tank may be ascertained from position sensor.

A control unit may be provided which is configured, in dependence on areceived indication of a pre-set mass of gas, to operate the gasdelivery system so that it delivers the pre-set mass of gas into theshock absorber. The received indication of a pre-set mass of gas may beascertained by the control unit by means of a look up table. The controlunit may be configured to receive an input from an input device thatreads data associated with the shock absorber in order to ascertain,from a look-up table, the pre-set mass of gas to be delivered into theshock absorber. The data read by the input device may for example be anidentification code, for example provided by a bar-code, RFID tag or thelike, which identifies the type of shock absorber. The same control unitmay be used to control the exhaust of gas from the chamber as is used tocontrol the supply of gas to the chamber.

The present invention also provides a control unit configured to performthe function of the control unit of the method or apparatus according toany aspect of the present invention as claimed or described herein,including any optional features relating thereto. The control unit maycomprise a programmable control unit programmed with appropriatesoftware. The present invention also provides a computer program productconfigured to cause, when the computer program is executed, aprogrammable control unit to form a control unit according to any aspectof the present invention as claimed or described herein, including anyoptional features relating thereto.

It will of course be appreciated that features described in relation toone aspect of the present invention may be incorporated into otheraspects of the present invention. For example, the method of theinvention may incorporate any of the features described with referenceto the apparatus of the invention and vice versa.

DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying schematic drawings ofwhich:

FIG. 1 shows an aircraft of a type that can be serviced by a servicingcart of either the first or second embodiment of the invention;

FIG. 2 shows a shock absorber of the aircraft of FIG. 1;

FIG. 3 shows part of the shock absorber of FIG. 2 illustrating themeasuring of an H-dimension;

FIG. 4 shows a two-stage shock absorber of a type that can be servicedby a servicing cart of either the first or second embodiment of theinvention;

FIGS. 5a and 5b show a lightweight portable servicing cart according toa first embodiment of the invention;

FIG. 6 shows schematically a first part of the servicing cart of FIGS.5a and 5b in use;

FIG. 7 shows a second part of the servicing cart of FIGS. 5a and 5b inuse;

FIG. 8 shows an interface unit of the second part shown in FIG. 7;

FIG. 9 is a flow-chart illustrating an example servicing process carriedout in accordance with the first embodiment of the invention;

FIGS. 10a and 10b show a portable automated-servicing cart according toa second embodiment of the invention;

FIG. 11 is a schematic showing in further detail the component parts ofthe servicing cart of FIGS. 10a and 10b ; and

FIG. 12 is a flow-chart illustrating an example servicing processcarried out in accordance with the second embodiment of the invention.

DETAILED DESCRIPTION

FIGS. 1 and 2 show an aircraft landing gear of a type suitable for usewith all of the illustrated embodiments of the present invention. Thus,there is shown in FIG. 1 an aircraft 101 comprising a pair of wings 106and a fuselage 105. The wings each carry an engine. The aircraft issupported on the ground by sets of landing gear assemblies comprising amain landing gear (MLG) 108 and a nose landing gear (NLG) 110. Eachlanding gear assembly is provided with suspension and shock absorptionfunctions, by means of a shock absorber assembly 102. The shock absorber102 is of a type where hydraulic fluid is in contact with Nitrogen gas(N₂), such that N₂ may be absorbed by the fluid. A schematicillustration of such a shock absorber 102 is shown in FIG. 4. Thus,there is shown a shock absorber 102 having a lower part (piston 104)arranged to slide within an upper (cylinder 103) so that the shockabsorber 102 is telescopic. There is a first upper chamber 111 in whichhydraulic fluid 135 and N₂ 121 are in fluid communication with eachother, so that there is at least one surface 133 of contact between thefluid 135 and gas 121. The upper chamber 111 is split into two separatecompartments 111 a, 111 b divided by a restrictive opening 113 arrangedto provide resistance for fluid flow between the compartments 111 a, 111b. There is also a second lower chamber 112 containing gas but nohydraulic fluid. The shock absorber 102 is thus in the form of atwo-stage shock absorber.

The gas in the upper chamber 111 acts as a gas spring thus providingpart of the suspension function of the shock absorber 102. Damping isprovided as a result of the hydraulic fluid being forced through therestrictive opening 113 as the piston 104 slides within the cylinder103. The gas in the lower chamber 112 acts as a secondary gas springwhich is of greater significance under relatively high loading. Thepressure in the lower chamber 112 is arranged to be significantly higherthan in the upper chamber 111 under low loading, so that the lowerchamber 112 only undergoes significant contraction when the pressure ofthe gas in the upper chamber 111 equals that in the lower chamber 112.There is a lower charging valve 115 associated with the lower chamber112. There is an upper charging valve 114 associated with the upperchamber 111.

The first embodiment of the present invention concerns servicing of anaircraft landing gear shock absorber 102 with the use of a lightweightportable servicing cart 117. The cart 117 is shown in FIGS. 5a and 5band comprises two primary parts, a deflation kit 118 (see FIG. 6) and aninflation kit 119 (see FIG. 7). The main components of the deflation kit118 (referred to below as the Deflation and Oil Replenishment Tool or“DORT”) are a vacuum pump 129, a hydraulic fluid trap 126 and a source128 of hydraulic fluid. The main components of the inflation kit arepressurised canisters of N₂ 120 (supplied separately from the cart 117shown in FIGS. 5a and 5b ), and a gas delivery system including acontrol unit 136.

FIG. 6 shows the “DORT” deflation kit 118 attached to a shock absorber102. There is included a quick release coupling 123 for connection tothe shock absorber 102 via a hose 122. A gas vent 130 is connected viavalves 125 and 127 and a fluid trap 126 to quick release coupling 123.Also connected to the quick release coupling 123 via the fluid trap 126and valve 125, is a vacuum pump 129. A pressure gauge 124 is providedbetween the quick release coupling 123 and valve 125. A hydraulic fluidreservoir 128 and associated fluid pump 131 are also connected to thequick release coupling 123.

FIG. 7 shows the inflation kit 119 attached to a shock absorber 102.There is included a quick release coupling 132 for connection to theshock absorber 102 via a hose 122. A further quick release coupling 134is provided as a gas inlet to allow for connection to a gas cylinder120. The gas inlet is connected via a gas flow regulator 137 to thequick release coupling 132 which acts therefore as a gas outlet, in use.The regulator 137 comprises a solenoid valve (not shown) and an orifice140, or other nozzle, so shaped as to create a choked flow of gas inuse. The regulator 137 also comprises an upstream pressure andtemperature sensor set 141 provided upstream of the orifice 140 and afurther, downstream, pressure and temperature sensor set 142 provideddownstream of the orifice 140. The sensor sets are connected to acontrol unit 136 which is, from the sensor readings, arranged tocalculate the mass of gas flowing through the orifice 140 per unit time.This can be achieved by means of pre-calibrating the gas flow regulator137.

The gas inlet coupling 134 is shown connected to a gas cylinder 120 viathe cylinder's gas regulation valve 139. Before and after the cylinder'sgas regulation valve 139 are provided pressure gauges 138.

The control unit 136 is connected to an interface unit 143 to allowoperator interaction. FIG. 8 shows the interface unit 143 in greaterdetail. Three light indicators 144 are provided: a green light 144 a toindicate a “ready” state, an amber light 144 b to indicate an “inprogress” state, and a red light 144 c to indicate a “fault” state.There is also provided a large “Start” press button 145, a user inputselector dial 146, and a text message display screen 147. The controlunit 136 comprises a computer processor, which is programmed withcomputer software held in memory readable by the computer processor andwhich, when executed, cause the control unit 136 to function as requiredby the present embodiment. There is also data held in memory accessibleby the control unit 136, such data representing the masses of gas thatare required for different given types of landing gear shock absorbersfor example.

A method of servicing a shock absorber 102 using the portable servicingcart 117 of the first embodiment will now be described.

As a first part of the process, deflation of the shock absorber 102 withthe aircraft 101 in the weight on wheels state is conducted. Inparticular, the upper chamber 111 in which gas 121 and hydraulic fluid135 are not separated is exhausted of all gas in the gaseous state, andfurther gas is then allowed to degas from the hydraulic fluid, which isthen topped up as necessary. This is achieved (after the aircraftparking brake has been applied and chocks have been installed to preventmovement) by conducting, in order, the steps set out below.

It will be appreciated that the process of the first embodiment isdescribed with reference to a 2-stage shock absorber 102. The sameequipment and process steps are able to be used for servicing asingle-stage shock absorber. If the shock absorber is single-stage steps2 to 4 and 17 to 19 are to be omitted, as indicated in FIG. 9 by meansof the steps shown in broken line. The steps conducted are as follows:

Connect Collar (see box 161 of FIG. 9)

Step 1—If required, install service collar (shown in FIGS. 6 and 7 asitem 5) on the lower piston 104.

Exhaust Lower Chamber 112 (see box 162 of FIG. 9), if Gear is Two-Stage

Step 2—Connect the Deflation and Oil Replenishment Tool (“DORT”) 118 viaquick release coupling 123 to the lower charging valve 115 via highpressure flexible hose 122.

Step 3—Open valves 115, 125 & 127 and vent the lower stage onto theservice collar. The gear will descend during this process.

Step 4—Disconnect the hose 122 from charging valve 115.

Exhaust Upper Chamber 111 (see box 163 of FIG. 9)

Step 5—Connect the Deflation and Oil Replenishment Tool (“DORT”) 118 viaquick release coupling 123 to the upper charging valve 114.

Step 6—Open upper charging valve 114 and let the upper stage N₂discharge via valves 125, 127 and venting orifice 130. The gear willagain descend during this process. Any hydraulic fluid that is carriedwith the gas, as may be expected, is collected in the fluid trap 126.

Degas Hydraulic Fluid (see box 164 of FIG. 9)

Step 7—Close valve 127 and start the vacuum pump 129. The pump is leftrunning for at least 1 hour which is typically long enough to degas asignificant proportion, if not substantially all, of the N₂ from thehydraulic fluid.

Step 8—After 1 hour, stop the vacuum pump 129 and close valve 125.

Check/Adjust Fluid Level (see box 165 of FIG. 9)

Step 9—Using the hand pump raise the pressure reading on the gauge 124to 5 bar. Hold at this pressure for a suitable length of time (a fewminutes or so) and if the pressure falls during this time use the handpump to restore the pressure to 5 bar. This will ensure that the upperchamber 111 is full of hydraulic fluid.

Step 10—Open Valve 125 and allow the pressure to return to zero onpressure gauge 124 and disconnect hose 122 from quick release coupling123. Valve 114 is left open. This then concludes the first part of theprocess, in which the Deflation and Oil Replenishment Tool is used.

After the first part of the process has been completed and the shockabsorber 102 is deflated, there is a known amount of degassed hydraulicfluid in the upper chamber 111. It is also known that the upper 111 andlower 112 chambers are exhausted of all gas. In the second part of theservice process the inflation kit 119 is used to inject a pre-set massof N₂ into the shock absorber chambers 111, 112. This is achieved byconducting, in order, the following steps, which follow on from thosesteps (1 to 10) listed above:

Inflate Upper Chamber 111 with Set Mass of Gas (see box 166 of FIG. 9)

Step 11—Connect a Gaseous N₂ Supply 120 with Regulator 139 to QuickRelease coupling 134, ensure that the N₂ supply is pressurised to atleast 200 bar as shown on gauge 138 before starting the charge process.

Step 12—Set the regulator 139 to show a pressure of 200 bar on pressuregauge 138.

Step 13—Connect the hose/line 122 to quick release coupling 132. It maybe necessary to take care not to disturb the hose connection at thecharging valve 114; it may be full of hydraulic fluid, but this would beas expected.

Step 14—On the interface panel the user selects the correct landing gearand stage chamber by means of using the user interface selector dial146, until the desired selection is displayed on the text display screen147.

Step 15—The user then presses the start button 145 and the upper stageis then charged under the control of the control unit 136. This isindicated by a continuous amber indication light. During this step thegear will lift.

Step 16—When charging of the upper stage is complete close up chargingvalve 114 and disconnect the hose 122 at 114.

Inflate Lower Chamber with Set Mass of Gas (see box 167 of FIG. 9), ifGear is Two-Stage

Step 17—Attach the flexible hose 122 to the lower charging valve 115 andensure the valve is open.

Step 18—Select the lower chamber for the correct gear using theinterface selector and press the start button 145. The lower chamber 112is then charged with gas under the control of the control unit 136. Thisis again indicated by a continuous amber indication light. During thisstep the gear may lift further.

Step 19—When charging is complete close up the lower stage chargingvalve 115.

Remove Collar (see box 168 of FIG. 9)

Step 20—Remove the service collar 5, if fitted in Step 1. The service isthen complete.

The controller performs internal checks in real-time to ensure that thecharge process follows an orderly path from start to finish and willstop, closing the solenoid and indicating a fault with the red light ifany of a number of failure conditions occur. If the control unit 136senses pressure or temperature readings that are unexpected, for examplehigher or lower than pre-set thresholds, or that a rate of change ofpressure or temperature is detected that is higher or lower than pre-setthresholds, then a fault may be assumed. Examples of particular faultconditions include, but are not limited to:

-   -   Lack of sufficient N₂ supply pressure or pressure falls below        acceptable limit for the charging process;    -   Lack of progress in charging the shock absorber 102 (rate of        pressure rise too low for the chamber) indicating a leak;    -   Charge rate too high indicating a blockage or restriction in the        charging line;    -   Failure of the solenoid valve to open, failure of either or both        pressure transducers.

In steps 14 and 18, the user selects the chamber position and type ofshock absorber/landing gear. Each selection is associated, in a look-uptable stored in memory accessible by the control unit 136, with acorresponding mass of N₂ that is to be delivered to an empty (of gas)chamber of the selected type.

In steps 15 and 18, upon pressing the start button the control unitopens the solenoid valve to allow N₂ to flow across orifice 140. Theupstream pressure and temperature sensor 141 and the downstream pressureand temperature sensor 142 are read continuously by the control unit.This information is used to derive the mass flow rate across orifice140. The mass flow rate is then integrated with respect to time (inreal-time) to obtain the mass of N₂ delivered to the shock absorber 102.When this delivered N₂ mass is equal to the correct mass as set in viewof the selection made with the control interface, the control unit 136switches off the solenoid valve halting the flow of gas. Such a means ofdelivery of N₂ is independent of pressures, loads on the gear,temperature and absorption of N₂ into the hydraulic fluid. There cantherefore be greater confidence than hitherto possible that the amountof N₂ in the shock absorbers 102 is appropriate.

The servicing cart 117 allows accurate servicing of the aircraft 101 ina weight on wheels state, taking into account N₂ absorption intoHydraulic fluid. Absorption of N₂ into the hydraulic fluid in a shockabsorber 102 results in a decrease in the mass of gas above thehydraulic fluid in a weight-on-wheels state, compared with (all otherthings being equal) the mass of gas above the hydraulic fluid, in theweight-off-wheels state (as a result of the gas having degassed from thehydraulic fluid). When the aircraft 101 takes off and the pressurewithin the shock absorber 102 is relieved it returns, with time, to itspre-landing state. After landing, when the aircraft 101 has come to arest, the amount of N₂ absorbed or otherwise contained in the hydraulicfluid within the shock absorber 102 will be a complex function of theaircraft 101 weight (and thus shock absorber 102 pressure), thesurrounding temperature and the extent to which the N₂ and fluid havebeen mixed during landing and taxiing. Thus the H-dimension of the shockabsorber 102 is variable with both service condition and aircraftactivity. The measurement of the H-dimension of the shock absorber 102(taken in a weight-on-wheels state) previously used to indicate the massof gas will often result in an underestimate of the mass of gas in theshock absorber 102. The accuracy of gas level checks based onH-dimension measurements can therefore be the subject of improvement bymeans of the presently described embodiment and/or such checks based onH-dimension measurements may be rendered redundant/unnecessary. Themethod of the embodiment also permits replenishment of hydraulic fluidwith the aircraft 101 in the weight on wheels state. The equipment onthe servicing cart 117 also permits the service process to be carriedout for any aircraft weight and centre of gravity condition and is notunduly sensitive to pressure, temperature and friction effects in theshock absorber. The equipment also permits accurate service of not onlyone-stage, but also two-stage, shock absorbers.

The second embodiment of the present invention concerns servicing of anaircraft landing gear shock absorber 102 with the use of a portableservicing cart 250 which is more automated than the first embodiment. Acart 250 is shown in FIGS. 10a and 10b and provides a fully integratedset of servicing equipment that can deflate gas from the shock absorber102, degas and replenish hydraulic fluid, and inject gas into the shockabsorber 102. The cart thus carries pressurised canisters of N₂ 251, asource 201 of hydraulic fluid, a gas delivery system 203 and a controlsystem 205 that controls the servicing process. FIG. 11 showsschematically the main parts of the cart 250 attached to a shockabsorber 102. There are two connections 260, 261 shown, which areprovided by hoses which connect between a connector on the cart and aconnector on the shock absorber 102. Thus there is a first connection260 between a connector on the cart and the connector associated withthe upper (main) chamber 111 of the shock absorber which contains bothhydraulic fluid and gas. There is a further connection 261 betweenanother connector on the cart and the connector associated with thesecond, lower, chamber 112 of the shock absorber which contains onlygas. Each connector on the cart has an associated oil trap 215, 216 forcollecting hydraulic fluid that may inadvertently be carried with gas. Apressure gauge/temperature sensor 209, 211 is provided next to each oiltrap 215, 216. Any hydraulic fluid caught by either oil trap is fed to arecycling tank 208.

A gas delivery system 203 includes a gas tank for capturing and storingN₂ gas from the shock absorber 102. A vacuum pump 206 is provided fordrawing N₂ out of solution from the fluid. The same gas tank is used tohold a predetermined mass of N₂ for delivery via the manifold 210 to theshock absorber 102. The gas tank is connected to a supply of N₂ for thispurpose. The gas tank has a hydraulically powered actuation system,which is capable of emptying the tank completely of the gas held withinit. The gas tank includes pressure and temperature sensors whichtogether with knowledge of the internal volume of the tank can be usedto calculate the mass of the N₂.

A hydraulic fluid reservoir 201 and associated fluid delivery system 202are also connected to the manifold 210 for supplying hydraulic fluid tothe shock absorber 102. The gas vacuum pump 206, gas delivery system203, hydraulic fluid delivery system 202, the manifold 210 and thevarious sensors are connected to a control unit 205, which is itselfconnected to a human interface unit 207. In this case, the humaninterface unit 207 is in the form of a touch sensitive display screen.

A method of servicing a shock absorber 102 using the portable servicingcart 250 of the second embodiment will now be described.

The semi-automatic process includes both manual steps (represented bybox 320) conducted before and after an otherwise automatic process(represented by box 310). At the start 321 of the servicing process, theoperator of the cart inputs details of the gear into theHuman-Machine-Interface (HMI) 207, attaches the supporting collar 5 tothe gear, and connects the two hoses to the shock absorber charge valves114, 115. These steps are represented by box 322 in FIG. 12. Theoperator then opens the charge valves (step 323) and then presses the‘Start’ button (step 324) on the Human-Machine-Interface 207 to beginthe automatic service process (represented by box 310). The system thenautomatically services the shock absorber in accordance with the stepsdescribed below (and illustrated in FIG. 12). It will be appreciatedthat Steps 1 and 6 are not applicable (and therefore not performed) forsingle-stage shock absorbers.

Step 1: Venting of the Lower Chamber 112 (box 301 in FIG. 12)

The N₂ in the lower chamber 112 is deflated via the manifold 210 intothe N₂ tank of the gas delivery system 203. The actuator in the tank canbe used at this stage to gradually increase the volume of the tank, thuscreating a negative pressure (relative to the pressure of gas in theshock absorber) thus drawing gas into the tank. It may be that below acertain pressure this process will stop, as the pressure in the tankequalises (and can not be lowered further by expanding further thevolume of the tank) with the pressure in the shock absorber 102. In sucha condition, the remaining gas in the shock absorber 102 (still underpressure) is then allowed to vent to atmosphere by means of operatingvalve 222. This valve 222 will be closed when the connecting transducer209 reads 1 bar absolute pressure, indicating that the lower chamber 112has been fully depressurised. Orifices (not shown in FIG. 11) areinstalled in the N₂ lines to limit the rate of depressurisation andthereby control the descent rate of the gear. The line contains an oiltrap 216 which is monitored for excessive leakage across the floating(separator) piston. Any recovered oil (hydraulic fluid) will be sent tothe recycle tank 208.

Step 2: Venting of Upper Unseparated Chamber 111 (box 302 in FIG. 12)

The N₂ in the upper-stage chamber 111 is captured and stored in the N₂tank of the gas delivery system 203 for later reuse in the serviceprocess. Below a certain pressure this process will stop, as thepressure in the tank equalises with the pressure in the shock absorber102. The remaining gas in the shock absorber 102 (still under pressure)is then allowed to vent to atmosphere by means of operating the valves(not shown) associated with the manifold 210. In the final stages ofdepressurisation (within the context of this step, step 2) oil isexpelled and collected in the oil trap 215 and sent to the recycle tank208.

It may be at this stage (or during step 2) that the special collar 5performs its function. The collar 5 serves two purposes. The firstpurpose is to act as an in-stop to precisely fix the internal volume ofthe main chamber 111, which enables the cart 250 to fill the mainchamber 111 with the correct volume of hydraulic fluid—see below. Thesecond purpose is that it will provide structural support to the gear,when deflated. It will facilitate the safe deflation, in conjunctionwith the control provided by the control unit 205, of the shock absorberchambers of N₂ so that the gear descends onto this collar 5.

Step 3: N₂ Degassing (box 303 in FIG. 12)

Residual N₂ contained in the hydraulic fluid following deflation of thegear is then removed by application of a vacuum pump 206. The vacuum ispressure-regulated to ensure that it does not fall below the vapourpressure of the hydraulic fluid. The vacuum pump 206 is operated toapply a partial vacuum (at an absolute pressure of say around 0.1 bar)to the shock absorber upper unseparated chamber 111. The vacuum pump 206is kept running for a set time period (about 60 minutes) to desorb N₂from the hydraulic fluid. The N₂ that degasses is vented to atmosphere.When this set period is complete vacuuming is stopped. On completion ofthis vacuum process there will be very little gas in the chamber and acertain amount of hydraulic fluid.

Step 4: Hydraulic Fluid Replenishment (box 304 in FIG. 12)

During steps 2 and 3, (the final stage of descent and de-pressurisation)hydraulic fluid will be forced out of the shock absorber 102 via thecharge/discharge line due to displacement action caused by gas bubbleevolution from the super-saturated liquid (rather like opening a bottleof carbonated drink). The action will create a hydraulic fluid deficitand one or more cavities in the landing gear chamber. Such cavities forma small minority of the space in the chamber (the rest being filled byhydraulic fluid) and will be in a condition close to a vacuum (i.e. atan absolute pressure of close to 0.1 bar). In Step 4, the hydraulicfluid is thus replenished under pressure.

The hydraulic delivery system 202 is operated to deliver hydraulic fluidfrom the on-board supply 201 to the upper-stage chamber 111 of the gear.The pumping is stopped when a set pressure is reached in the deliveryline, as measured by the connecting transducer 211. This step (step 4)occurs immediately after the vacuum process (step 3) has been completed,so it can be assumed that the gas contained in the shock absorber 102 isnegligible (i.e. there is a partial vacuum such that, as a practicalmatter, all gas will have been exhausted from the chamber). By fillingthe shock absorber 102 with hydraulic fluid to the set pressure (whichis at about 5 bar, say) from such a vacuum state, it can be known withreasonably high accuracy the volume of hydraulic fluid and the amount(negligible) of gas in the shock absorber 102. The previous vacuum statethus facilitates correct hydraulic fluid intake. The set deliverypressure is chosen to ensure that hydraulic fluid fills the availablespace and that there is sufficient pressure to overcome line resistanceand compress any pre-existing cavities to a negligible volume withoutcausing any gear movement or change of volume.

Step 5: N₂ Replenishment Upper-Stage Chamber 305 in FIG. 12)

The mass of N₂ required in the upper-stage chamber 111 is retrieved froma lookup table stored in the memory of the control unit 205. This targetmass will be made up from the N₂ recovered in the gas tank and (as muchas is required) gas added from the on-board N₂ supply. The volume of thegas tank is known. With knowledge of the pressure and temperature of thegas in the tank it is then possible to know with relative high accuracythe mass of N₂ in the tank. The tank can thus be filled until thepressure and temperature are indicative of the mass of gas in the tankhaving reached the target mass. This mass of N₂ is then injected by theN₂ delivery system 203 into the upper-stage chamber 111, by means ofactuating the hydraulically powered actuation system.

Pressure checks are performed to ensure that these remain within safelimits so that in the event of a blockage or loss of pressure due toleakage, such an event may be quickly detected.

Step 6: N₂ Replenishment in Lower-Stage Chamber 112 (box 306 in FIG. 12)

For two-stage gears the mass of N₂ required in the lower separatedchamber 112 will be retrieved from a lookup table stored in the memoryof the control unit. This pre-set mass of N₂ will then be received inthe tank, and then injected by the N₂ delivery system 203 into the lowerchamber 112.

Once Step 6 is complete, the control unit 205 causes (step 307) thedisplay of an on-screen message to let the operator know that theautomated part 310 of the service process is finished.

The manual operator then closes the shock absorber charge valves 114,115 (step 325) and detaches the hose 122 (step 326) and supportingcollar 5 from the gear (also step 326). Step-by-step instructions forthis are provided by means of an appropriate video and audio commentaryon the HMI. Proximity sensors (not shown) are provided to indicate thatall hoses and collars are correctly stowed before the cart indicatesthat the process is complete (also indicated by an appropriate displayon the HMI).

During the process, the control unit 205 receives temperature andpressure signals from various sensors and monitors such signals forunexpected values.

The servicing process may thus be conducted by a single manual operator.The operator need only connect and disconnect certain attachments to thelanding gear at the start and at the end of the automated serviceprocess. No further intervention is required unless a fault occurs.

It will be seen that with the use of the cart 250 of the secondembodiment, pressurised N₂ is recovered into a special tank 208. Reusingthe N₂ in this way limits the rate at which the cart's N₂ supply is usedup, improving availability of the cart 250 and reducing the frequency ofN₂ bottle replacement. This cart 250 contains a N₂ boost pump 204 tofurther reduce the frequency of N₂ bottle replacement, by means of usingas much N₂ from the bottles as possible. The boost pump achieves this bymeans of enabling N₂ to be supplied from the bottles at below servicepressure (i.e. using the boost pump to pump out N₂ from the bottles).

The automated servicing cart 250 offers several benefits, which will nowbe explained.

The automated servicing cart 250 enables accurate weight on wheelsservicing of both N₂ and hydraulic fluid. This may reduce operator costsas N₂ and hydraulic fluid replenishment can be carried out on rampduring turn-around of the aircraft 101 (i.e. the aircraft will not needto be taken out of revenue service for the shock absorber N₂ andhydraulic fluid levels to be restored to within acceptable operatinglimits).

The automated servicing cart 250 significantly reduces the number ofmanual actions required by the maintainer. This may reduce thelikelihood of human error resulting in mis-serviced gears.

The process accounts for N₂ absorption in unseparated chambers and soremoves potential service errors associated with this phenomenon. Thiswill reduce the risk of a mis-serviced condition.

Some gear have more than one inlet/exit valve on a given chamber toallow through-flushing of hydraulic fluid, which could be used toprovide a means of mitigating N₂ absorption. The design of the servicingcart 250 does not need or use these additional valves as vacuuming isconsidered a more effective and efficient means for dealing with therecognised N₂ absorption issue.

The embodiments described will work equally well on separated andunseparated chambers as the actual mass of gas injected will beinvariant of the actual chamber conditions.

Whilst the present invention has been described and illustrated withreference to particular embodiments, it will be appreciated by those ofordinary skill in the art that the invention lends itself to manydifferent variations not specifically illustrated herein. By way ofexample only, certain possible variations will now be described.

Aspects of the first embodiment may be used in the second embodiment andvice versa.

The servicing cart 250, of the second embodiment for example, could bewirelessly connected to the internet so that data could be downloadedfor a given new gear type. Such data may include details of the pre-setmass of gas required for the given new gear type. Also, such a wirelessconnection could be used to send data from the servicing cart for thepurpose of health monitoring. For example, data may include frequency ofservicing of the particularly landing gear and/or the amount of gasexhausted from a given landing gear shock absorber.

The servicing cart 250, of the second embodiment for example, could readthe gear type using a number of automated methods including a bar-codescanner, Radio Frequency Identification (RFID) tag or Optical CharacterRecognition of a digital image of the Part/Serial Number plate on thegear. This could then be used to set the parameters for the gear beingserviced.

The correct mass of N₂ could be injected using a hydraulically actuatedvariable volume tank by knowing and then monitoring the volume, pressureand temperature until such time as the differences between startingconditions and end conditions are indicative of having injected thecorrect mass of gas is provided for the given chamber.

The apparatus may be arranged to service a single type of suspensionstrut or a plurality of different suspension struts that may includesingle or multiple stage struts.

Some aircraft may comprise landing gear with different struts, forexample, between the nose landing gear and the main landing gear.

The servicing cart 250 described above is a mobile system and compriseswheels. There may also be provided a braking system, a handle for manualmovement or means for powering the wheels, a steering system, a powersupply or protective bodywork systems. The servicing cart 250 may becoupled to or integrated with another ground system associated withaircraft servicing.

It will be understood by those skilled in the art that the processingfunctionality of the apparatus that embodies a part or all of thepresently described embodiments of the invention (for example thecontrol unit described herein) may be a general purpose device havingsoftware arranged to provide a part or all of such functionality. Thedevice could be a single device or a group of devices and the softwarecould be a single program or a set of programs. Furthermore, any or allof the software used to implement the invention can be communicated viaany suitable transmission or storage means so that the software can beloaded onto one or more devices.

To ensure that the correct process is followed, interactive video couldbe used to provide the operator with hose and service collar attachmentinstructions, and/or closing up and disconnection procedures.

The servicing cart may also be of benefit when servicing separated shockabsorbers. For example, an embodiment may provide a portable apparatusfor servicing a shock absorber on a landing gear assembly of an aircraftwhen the aircraft is in a weight on wheels configuration, the shockabsorber comprising at least one chamber containing hydraulic fluid andat least one chamber containing gas, not necessarily in fluidcommunication with the fluid. Such an apparatus may comprise a source ofgas, a source of hydraulic fluid, and a gas delivery system fordelivering a pre-set mass of gas into the shock absorber.

Where in the foregoing description, integers or elements are mentionedwhich have known, obvious or foreseeable equivalents, then suchequivalents are herein incorporated as if individually set forth.Reference should be made to the claims for determining the true scope ofthe present invention, which should be construed so as to encompass anysuch equivalents. It will also be appreciated by the reader thatintegers or features of the invention that are described as preferable,advantageous, convenient or the like are optional and do not limit thescope of the independent claims. Moreover, it is to be understood thatsuch optional integers or features, whilst of possible benefit in someembodiments of the invention, may not be desirable, and may therefore beabsent, in other embodiments.

The invention claimed is:
 1. A method of servicing a shock absorber on alanding gear assembly of an aircraft, the shock absorber comprising atleast one chamber containing both hydraulic fluid and a gas in fluidcommunication with each other, wherein the method comprises thefollowing steps all conducted whilst the weight of the aircraft is atleast partially supported by the landing gear assembly: exhausting allgas from the chamber; degassing at least some dissolved gas from thehydraulic fluid; ensuring that one or more set criteria, which depend onthe amount of hydraulic fluid in the chamber are met; adjusting theamount of hydraulic fluid in the chamber; and, delivering a pre-set massof gas into the chamber.
 2. A method according to claim 1, whereinduring the step of exhausting all gas from the chamber, the volume ofthe chamber reduces so that the shock absorber contracts, and the methodfurther includes a step of arranging a stop to resist furthercontraction of the shock absorber.
 3. A method according to claim 1,wherein before the step of exhausting all gas from the chamber iscompleted, arranging a collar around the landing gear assembly, thecollar bearing at least some of the load that would otherwise besustained by the shock absorber.
 4. A method according to claim 1,wherein the step of degassing at least some dissolved gas from thehydraulic fluid includes a step of actively lowering the pressure withinthe chamber.
 5. A method according to claim 1, wherein the step ofdelivering a pre-set mass of gas into the chamber includes deliveringthe pre-set mass via a regulator that is configured to measure orregulate the rate of injection of mass of gas per unit time.
 6. A methodaccording to claim 1, further comprising providing a control unit whichis configured to control processes effected by the method including oneor more of the step of exhausting gas from the chamber, the step ofdegassing dissolved gas from the hydraulic fluid, the step of ensuringthat said set criteria are met, and the step of delivering a pre-setmass of gas into the chamber.
 7. A method according to claim 1, furthercomprising providing a control unit and wherein during at least one ofthe steps of the method the control unit monitors inputs includinginputs concerning both temperature and pressure from sensors forconditions suggestive of a fault.
 8. A method of servicing a shockabsorber on a landing gear assembly of an aircraft whilst the weight ofthe aircraft is at least partially supported by the landing gearassembly, wherein the shock absorber comprises at least one chambercontaining both hydraulic fluid and a gas in fluid communication witheach other, comprising: exhausting all gas from the at least onechamber; degassing at least some dissolved gas from the hydraulic fluid;ensuring that one or more set criteria, which depend on the amount ofhydraulic fluid in the chamber are met; adjusting the amount ofhydraulic fluid in the chamber; providing a gas delivery system; and,delivering a pre-set mass of gas into the chamber with the gas deliverysystem.
 9. A method according to claim 8, further comprising: providinga vacuum pump; and, exhausting gas from the at least one chamber usingthe vacuum pump.
 10. A method according to claim 9, further comprising:providing a fluid trap; and, collecting hydraulic fluid that may flowout of the at least one chamber as the gas is exhausted using the fluidtrap.
 11. A method according to claim 8, further comprising: providing apump; and, delivering hydraulic fluid to the shock absorber from thereservoir using the pump.
 12. A method according to claim 3, wherein thecollar stops contraction by sustaining compressive loads of the samemagnitude as those sustained by the shock absorber in a weight-on-wheelsstate.
 13. A method according to claim 8, further comprising: providinga regulator; and, configuring the regulator to measure or regulate therate of injection of mass of gas per unit time.
 14. A method accordingto claim 8, further comprising a source of hydraulic fluid, and addinghydraulic fluid to the chamber from the source of hydraulic fluid.